Acoustic resonators, and particularly bulk acoustic wave (BAW) resonators, are used in many high-frequency communication applications. In particular, BAW resonators are often employed in filter networks that operate at frequencies above 1.5 GHz and require a flat passband. Moreover, BAW resonators have exceptionally steep filter skirts and squared shoulders at the upper and lower ends of a passband. BAW resonators also provide excellent rejection outside of the passband. Further still, BAW-based filters also have relatively low insertion loss, tend to decrease in size as the frequency of operation increases, and are relatively stable over wide temperature ranges. As such, BAW-based filters are desirable for many third generation (3G) and fourth generation (4G) wireless devices and are destined to dominate filter applications for fifth generation (5G) wireless devices. Most of these wireless devices support cellular, wireless fidelity (Wi-Fi), Bluetooth, and/or near field communications on the same wireless device, and as such, pose extremely challenging filtering demands. While these demands keep raising the complexity of the wireless devices, there is a constant need to improve the performance of BAW resonators and BAW-based filters.
Harmonics are generated when power passes through a BAW filter. The second harmonic (H2) generation in a BAW filter is commonly referred to as “H2 self-generation.” Traditional techniques to minimize H2 self-generation such as apodization and/or use of border rings may not adequately suppress H2 levels within power amplifier multiplexer networks that employ BAW-based filters. Thus, there remains a need for structures that reduce H2 levels within power amplifier multiplexer networks.